CLHS: Issue STREAM-CAPABILITIES Writeup

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## Issue STREAM-CAPABILITIES Writeup

    Issue:         STREAM-CAPABILITIES
    References:    Standard streams (pp. 327-329)
    Category:      ADDITION
    Edit history:  Version 1 by Pierson 5-Jul-88, add redesign per Pitman
                   Version 2 by Moon, 10-May-89, remove controversial parts
                   Version 3 by Moon, 12-May-89, improve wording and examples

    Problem description:

      Programs cannot currently distinguish between interactive use and batch
      (or background) use without using implementation-dependent extensions.
      For example, there is currently no way to tell whether it is useful to
      ask a question when an unexpected situation is encountered.

      Note: earlier versions of this issue tried to solve another problem
      as well.  See discussion section.

    Proposal (STREAM-CAPABILITIES:INTERACTIVE-STREAM-P):

      Add the following function:

        INTERACTIVE-STREAM-P stream

        Returns T if the stream is interactive, otherwise NIL.  Signals
        an error of type TYPE-ERROR if the argument is not a stream.

        The precise meaning of INTERACTIVE-STREAM-P is implementation
        dependent, and may depend on the underlying operating system.  However
        the general intent is to distinguish between interactive and batch (or
        background or command-file) operations.

        Some examples of the things that might identify a stream as interactive
        include:
           1. The stream is connected to a person (or equivalent) in
              such a way that the program can prompt for information and
              expect to receive different input depending on the prompt.
           2. The program is expected to prompt for input and support
              "normal input editing".
           3. READ-CHAR might hang waiting for the user to type something
              instead of quickly returning a character or EOF.

        *TERMINAL-IO* might or might not be interactive.

    Examples:

      (when (> measured limit)
        (let ((error (round (* (- measured limit) 100)
                            limit)))
          (unless (if (interactive-stream-p *query-io*)
                      (yes-or-no-p "The frammis is out of tolerance by ~D%.~@
                                    Is it safe to proceed? " error)
                      (< error 15))  ;15% is acceptable
            (error "The frammis is out of tolerance by ~D%." error))))

    Rationale:

      INTERACTIVE-STREAM-P has been proposed several times and is clearly
      needed by any program that alters its behavior depending on whether
      it is interacting with a user or running in a "batch" mode.

    Current practice:

      Most implementations have this feature already, often under a
      different name.

    Cost to Implementors:

      Implementations will have to support this new function.  Correct support
      will require some thought for each operating system supported.

    Cost to Users:

      None, this is an upward-compatible extension.

    Cost of non-adoption:

      Less featureful language.

    Performance impact:

      None.

    Benefits:

      More featureful language.

    Esthetics:

      More featureful language.

    Discussion:

      It's not possible to make a specific definition of "interactive" that
      applies to all operating systems, we concluded from earlier discussion.
      However, "everyone knows" appoximately what it means.  Hence the idea
      of describing it by example rather than defining it.

      Note that some proposed features for telling whether two streams are
      connected to the same source or sink of information have been removed
      from this version of the proposal.  These were the functions
      STREAM-SAME-SOURCE-P, STREAM-SAME-DESTINATION-P, STREAM-SOURCE-ID-LIST,
      and STREAM-DESTINATION-ID-LIST.  These could be revived in another
      proposal if desired, but Moon thought INTERACTIVE-STREAM-P was
      important and didn't want it to be lost due to controversy over these
      unrelated functions.

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